A proposal for the identification, isolation and localization of surface membrane receptors in boar spermatozoa involved in egg interactions during in vitro fertilization is described. Several approaches for the identification of receptors will be used. The first approach uses purified plasma membranes from different regions of the spermatozoon, and protein extracts therefrom, to alter the penetration time, binding, and fusion of capacitated spermatozoa with eggs. The second approach uses radiolabelled plasma membranes and protein extracts to measure specific binding interactions with egg investments particularly zona pellucida. The third approach uses antibodies to sperm plasma membrane, either directly reacted with sperm or isolated zona preparations, as a means of identifying surface receptor antigens. The location of receptors, identified and isolated by these procedures, on the plasma membrane of intact spermatozoa will be determined using techniques of immunofluorescence light microscopy and transmission electron microscopy.